Electronic device using group iii nitride semiconductor and its fabrication method

ABSTRACT

The present invention discloses an electronic device formed of a group III nitride. In one embodiment, a substrate is fabricated by the ammonothermal method and a drift layer is fabricated by hydride vapor phase epitaxy. After etching a trench, p-type contact pads are made by pulsed laser deposition followed by n-type contact pads by pulsed laser deposition. The bandgap of the p-type contact pad is designed larger than that of the drift layer. Upon forward bias between p-type contact pads (gate) and n-type contact pads (source), holes and electrons are injected into the drift layer from the p-type contact pads and n-type contact pads. Injected electrons drift to the backside of the substrate (drain).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Pat. App. 62/438,900 filed onDec. 23, 2016 and entitled “ELECTRONIC DEVICE USING GROUP III NITRIDESEMICONDUCTOR AND ITS FABRICATION METHOD”, inventors Tadao Hashimoto andDaisuke Ueda.

This application is related to the following patent applications:

PCT Patent Application Serial No. PCT/US2017/058775, by Tadao Hashimotoand Daisuke Ueda, entitled “ELECTRONIC DEVICE USING GROUP III NITRIDESEMICONDUCTOR AND ITS FABRICATION METHOD”, filed on Oct. 27, 2017,attorneys docket number SIXPOI-026WO;

PCT Utility Patent Application Serial No. US2005/024239, filed on Jul.8, 2005, by Kenji Fujito, Tadao Hashimoto and Shuji Nakamura, entitled“METHOD FOR GROWING GROUP III-NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIAUSING AN AUTOCLAVE,” attorneys' docket number 30794.0129-WO-01(2005-339-1);

U.S. Utility patent application Ser. No. 11/784,339, filed on Apr. 6,2007, by Tadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled“METHOD FOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS INSUPERCRITICAL AMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,”attorneys docket number 30794.179-US-U1 (2006-204), which applicationclaims the benefit under 35 U.S.C. Section 119(e) of U.S. ProvisionalPatent Application Ser. No. 60/790,310, filed on Apr. 7, 2006, by TadaoHashimoto, Makoto Saito, and Shuji Nakamura, entitled “A METHOD FORGROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS IN SUPERCRITICALAMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,” attorneysdocket number 30794.179-US-P1 (2006-204);

U.S. Utility Patent Application Ser. No. 60/973,662, filed on Sep. 19,2007, by Tadao Hashimoto and Shuji Nakamura, entitled “GALLIUM NITRIDEBULK CRYSTALS AND THEIR GROWTH METHOD,” attorneys docket number30794.244-US-P1 (2007-809-1) and issued as U.S. Pat. No. 8,253,221 and9,243,344;

U.S. Utility patent application Ser. No. 11/977,661, filed on Oct. 25,2007, by Tadao Hashimoto, entitled “METHOD FOR GROWING GROUP III-NITRIDECRYSTALS IN A MIXTURE OF SUPERCRITICAL AMMONIA AND NITROGEN, AND GROUPIII-NITRIDE CRYSTALS GROWN THEREBY,” attorneys docket number30794.253-US-U1 (2007-774-2) and issued as U.S. Pat. No. 7,803,344;

Allowed U.S. Utility patent application Ser. No. 12/392,960, filed onFeb. 25, 2009, by Tadao Hashimoto, Edward Letts, Masanori Ikari,entitled “METHOD FOR PRODUCING GROUP III-NITRIDE WAFERS AND GROUPIII-NITRIDE WAFERS,” attorneys docket number SIXPOI-003US;

U.S. Utility patent application Ser. No. 12/455,760, filed on Jun. 4,2009, by Edward Letts, Tadao Hashimoto, Masanori Ikari, entitled“METHODS FOR PRODUCING IMPROVED CRYSTALLINITY GROUP III-NITRIDE CRYSTALSFROM INITIAL GROUP III-NITRIDE SEED BY AMMONOTHERMAL GROWTH,” attorneysdocket number SIXPOI-002US and issued as U.S. Pat. No. 8,728,234;

U.S. Utility patent application Ser. No. 12/455,683, filed on Jun. 4,2009, by Tadao Hashimoto, Edward Letts, Masanori Ikari, entitled“HIGH-PRESSURE VESSEL FOR GROWING GROUP III NITRIDE CRYSTALS AND METHODOF GROWING GROUP III NITRIDE CRYSTALS USING HIGH-PRESSURE VESSEL ANDGROUP III NITRIDE CRYSTAL,” attorneys docket number SIXPOI-005US andissued as U.S. Pat. No. 8,236,237;

U.S. Utility patent application Ser. No. 12/455,181, filed on Jun. 12,2009, by Tadao Hashimoto, Masanori Ikari, Edward Letts, entitled “METHODFOR TESTING III-NITRIDE WAFERS AND III-NITRIDE WAFERS WITH TEST DATA,”attorneys docket number SIXPOI-001US and issued as U.S. Pat. No.8,357,243, 8,585,822, and 8,557,043;

U.S. Utility patent application Ser. No. 12/580,849, filed on Oct. 16,2009, by Tadao Hashimoto, Masanori Ikari, Edward Letts, entitled“REACTOR DESIGN FOR GROWING GROUP III NITRIDE CRYSTALS AND METHOD OFGROWING GROUP III NITRIDE CRYSTALS,” attorneys docket numberSIXPOI-004US;

U.S. Utility patent application Ser. No. 13/781,509, filed on Feb. 28,2013, by Tadao Hashimoto, entitled “COMPOSITE SUBSTRATE OF GALLIUMNITRIDE AND METAL OXIDE,” attorneys docket number SIXPOI-012US andissued as U.S. Pat. No. 9,224,817 and 9,431,488;

U.S. Utility patent application Ser. No. 13/781,543, filed on Feb. 28,2013, by Tadao Hashimoto, Edward Letts, Sierra Hoff entitled “ABISMUTH-DOPED SEMI-INSULATING GROUP III NITRIDE WAFER,” attorneys docketnumber SIXPOI-013US and issued as U.S. Pat. No. 9,255,342 and 9,435,051;

U.S. Utility patent application Ser. No. 13/833,443, filed on Mar. 15,2013, by Tadao Hashimoto, Edward Letts, Sierra Hoff entitled “METHOD OFGROWING GROUP III NITRIDE CRYSTALS,” attorneys docket numberSIXPOI-014US1 and issued as U.S. Pat. No. 9,518,340;

U.S. Utility patent application Ser. No. 13/834,015, filed on Mar. 15,2013, by Tadao Hashimoto, Edward Letts, Sierra Hoff entitled “METHOD OFGROWING GROUP III NITRIDE CRYSTALS,” attorneys docket numberSIXPOI-014US2 and issued as U.S. Pat. No. 9,202,872;

U.S. Utility patent application Ser. No. 13/834,871, filed on Mar. 15,2013, by Tadao Hashimoto, Edward Letts, Sierra Hoff entitled “GROUP IIINITRIDE WAFER AND ITS PRODUCTION METHOD,” attorneys docket numberSIXPOI-015US1 and issued as U.S. Pat. No. 9,543,393;

U.S. Utility patent application Ser. No. 13/835,636, filed on Mar. 15,2013, by Tadao Hashimoto, Edward Letts, Sierra Hoff entitled “GROUP IIINITRIDE WAFER AND ITS PRODUCTION METHOD,” attorneys docket numberSIXPOI-015US2 and issued as U.S. Pat. No. 8,921,231;

U.S. Utility patent application Ser. No. 13/798,530, filed on Mar. 13,2013, by Tadao Hashimoto, entitled “GROUP III NITRIDE WAFERS ANDFABRICATION METHOD AND TESTING METHOD,” attorneys docket numberSIXPOI-016US;

U.S. Utility patent application Ser. No. 14/329,730, filed on Jul. 23,2014, by Tadao Hashimoto, entitled “ELECTRONIC DEVICE USING GROUP IIINITRIDE SEMICONDUCTOR AND ITS FABRICATION METHOD,” attorneys docketnumber SIXPOI-017US and issued as U.S. Pat. Nos. 9,466,481, 9,685,327,9,305,772, and 9,349,592;

and all of the above patents and applications are incorporated byreference herein in their entirety as if put forth in full below.

BACKGROUND Field of the Invention

The invention relates to a semiconductor electronic device primarilyused for high-power and/or high-frequency electric/electronic circuit.More specifically, the invention relates to transistors formed of groupIII nitride semiconductor.

Description of the Existing Technology.

(Note: This patent application refers several publications and patentsas indicated with numbers within brackets, e.g., [x]. A list of thesepublications and patents can be found in the section entitled“References.”)

Gallium nitride (GaN) and its related group III nitride alloys are thekey semiconductor material for various electronic devices such as powerswitching transistors. Despite the fact that the maximum performance ofGaN theoretically predicted with Baliga's Figure of Merit (BFOM) exceedsthat of silicon carbide (SiC) by approximately 5-fold, the lack oflow-cost and low-defect GaN substrates impedes development of GaN-basedpower switching transistors having their full potential. Currently, themajority of GaN-based devices are fabricated using a group III nitridefilm grown heteroepitaxially on a heterogeneous substrate, such assilicon, SiC and sapphire. However, heteroepitaxial growth of group IIInitride results in highly defected or even cracked films. Typicaldefects in group III nitride heteroepitaxial films are threadingdislocations at the level of 10⁹ cm⁻² along the growth direction.Because threading dislocations and/or cracks propagate verticallythrough a substrate, these defects can become current leakage paths whenhigh-voltage is applied vertically (i.e. along the substrate's growthdirection, typically described as the substrate's thickness).

Therefore, at this moment, GaN-based electronic devices are practicallylimited to horizontal devices such as high-electron mobility transistors(HEMT), which utilize current flow along the lateral direction near asubstrate's major surface. Since the electric current passes through athin film in such horizontal devices, a horizontal device requires alarge major-surface area to realize high-current (i.e. high-power)devices. In addition, all contacts are located on one side of thedevice, which causes device size to be larger than a verticalconfiguration. Due to these limitations, it is quite challenging toattain high-power devices in horizontal configuration of group IIInitride semiconductors.

To overcome the issues in horizontal group III nitride devices, peoplehave started to develop vertical type electronic devices using GaNsubstrates. Vertical high-power switching devices require normally-offoperation, low-series resistance, high-breakdown voltage, fast switchingspeed, high efficiency and low cost. However, people have notdemonstrated viable vertical high-power transistors using groupIII-nitride semiconductors due to many technical challenges such asdifficulties in obtaining low-cost, low-defect substrates, growinghigh-purity drift layers with accurate control of carrier concentration,and/or fabricating high-quality buried portion of group III-nitridecrystals for controlling current paths. To achieve commercially viablehigh-power vertical devices with group III-nitride semiconductors, aselection of substrate, design of device structures and a selection offabrication method must be carefully considered.

SUMMARY OF THE INVENTION

The present invention provides a vertical-type electronic device havinga group III nitride substrate. The present invention also providesvarious methods of forming an electronic device.

In one embodiment, a vertical-type electronic device has a GaN substrateor other substrate such as silicon having high-electron concentration(e.g. higher than about 5×10^(˜) cm⁻³) and low dislocation density (e.g.less than 5×10⁵ cm⁻² ). This vertical electronic device in which thesource and drain are typically but not exclusively on opposite sides ofthe substrate has low series resistance and high reliability compared tothe typical horizontal electronic device. The drift layer of the devicemay be fabricated to have an electron concentration of e.g. less thanabout 5×10¹⁶ cm⁻³. Also, the concentration of carbon in the drift layeris optionally reduced through selection of fabrication method andconditions as explained in more detail below.

To inject electrons into the drift layer so that the device can turn on,p-type contact pads and optionally n-type contact pads are attached tothe same side of the drift layer. The p-type contact pads and n-typecontact pads are not attached together directly.

The vertical-type electronic device can be configured as a power diodeby forming p-type and backside contacts but not n-type contact pads.

The vertical-type electronic device can be configured as a transistor byforming p-type, n-type, and backside contacts on the group III nitridesubstrate. The backside contact is the drain of the transistor ortransistors, the Ohmic contacts on the n-type contact pads are each asource for their respective transistors, and the Ohmic contacts on thep-type contact pads are each a gate for their respective transistors.

The electron concentration in the drift layer is designed to be lowenough to prevent current flow from drain to source under zero biasbetween the source and drain, which achieves normally-off operation.Under forward bias between the source and gate, holes are injected fromthe p-type contact pads (gate) to the drift layer and electrons areinjected from the n-type contact pads (source) to the drift layer. Theinjected electrons drift toward the drain so that the current flows fromdrain to source. To optimize the performance of the device, theconfiguration of the contact pads, carrier concentrations of eachlayer/portion, alloy compositions of each layer/portion and impurityconcentrations of each layer/portion are carefully designed as describedmore fully below.

In still further examples, the invention provides a vertical electronicdevice comprising an n-type contact pad, a p-type contact pad, a driftlayer having a thickness, a drain, and a substrate. The drift layer ispositioned between the n-type contact pad and the drain so that anelectrical current flows from the n-type contact pad through thethickness of the drift layer and to the drain. The n-type contact pad isadjacent to the p-type contact pad, and the n-type contact pad and thep-type contact pad are positioned sufficiently closely to one another onthe substrate so that a depletion region in the drift layer created bythe p-type contact pad prevents current flow from the n-type contact padon the drift layer to the drain when no voltage is applied between then-type contact pad and the p-type contact pad. This configurationprevents current leakage in a normally-off transistor designed to handlehigh power switching.

Any of the electronic devices above can be configured with the drain onthe opposite side of the substrate from the n-type contact pad, thep-type contact pad, and the drift layer or on the same side of thesubstrate as the n-type contact pad, the p-type contact pad, and thedrift layer.

The p-type contact pad in any device above may have a bandgap that islarger than a bandgap of the drift layer.

Examples of what are disclosed include the following:

(1) An electronic device having a substrate, a drift layer, and a p-typecontact pad, where the substrate has (a) a dislocation density less than5×10⁵ cm⁻² and (b) an electron concentration higher than 5×10¹⁸ cm⁻³;the drift layer has an electron concentration lower than 5×10¹⁶ cm⁻³;and the p-type contact pad has a hole concentration higher than 1×10¹⁷cm⁻³. This electronic device may have the p-type contact pad and ann-type contact pad positioned sufficiently close to one another on thesubstrate so that a depletion region in the drift layer created by thep-type contact pad prevents current flow from the n-type contact pad onthe drift layer to the Ohmic contact when no voltage is applied betweenthe n-type contact pad and the p-type contact pad. The p-type contactpad of this electronic device may, alternatively to the positioning ofthe p-type and n-type contact pads or additionally to it, have a bandgapthat is larger than a bandgap of the drift layer. A substrate ofGa_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1) (particularly GaN) or othersubstrate such as sapphire or silicon may be used in the device.

(2) An electronic device as in (1) above, where the p-type contact padhas a hydrogen concentration less than one hundredth of a hydrogenconcentration of the substrate.

(3) An electronic device having (a) a semi-insulating substrate ofGa_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1) (e.g. with resistivity ofat least 1 kΩ cm) having a dislocation density less than 5×10⁵ cm⁻²; (b)a drain layer of Ga_(1-x5-y5)Al_(x5)In_(y5)N (0≤x5≤1, 0≤y5≤1) and adrift layer of Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1) on the sameside of the substrate; (c) a p-type contact pad ofGa_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) in electrical contact withthe drift layer; (d) an Ohmic contact in electrical contact with thep-type contact pad; and (e) an n-type contact pad ofGa_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) in electrical contact withthe drift layer and spaced apart from the p-type contact pad.

(4) A method of fabricating an electronic device that comprises;

-   -   (a) growing a drift layer of Ga_(1-x2-y2)Al_(x2)In_(y2)N        (0≤x2≤1, 0≤y2≤1) by vapor phase epitaxy on a first side of a        substrate of Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1);    -   (b) forming p-type contact pads of Ga_(1-x3-y3)Al_(x3)In_(y3)N        (0≤x3≤1, 0≤y3≤1) on the drift layer by a deposition method which        does not use a hydrogen-containing source (e.g. the source is        not a hydride); and    -   (c) forming an Ohmic contact on a second side of the substrate.

The first or lower drift layer of any device or method herein may bee.g. Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1), and the second driftlayer may be Ga_(1-x6-y6)Al_(x6)In_(y6)N (0≤x6≤1, 0≤y6≤1). x2 and x6 maybe the same value or may be different, and y2 and y6 may be the samevalue or may be different.

These devices and methods and others are readily perceived by those inthe field from the disclosure herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is one example of an electronic device formed using a group IIInitride substrate.

In the figures each number represents the following:

-   -   1. Homoepitaxial substrate    -   2. Backside contact    -   3. Drift layer    -   3 a. A depletion region created inside the drift layer    -   4. p-type contact pads    -   5. Ohmic contact to the p-type contact pads    -   6. n-type contact pads    -   7. Ohmic contact to the n-type contact pads

FIG. 2 is a top view of one example of an electronic device whoseprofile is depicted in FIG. 1. FIG. 2 is one possible view along lineA-A of FIG. 1. Each number represents the following:

-   -   10. A region for drift layer    -   10 a. A depletion region created inside the drift layer by the        p-type contact pad    -   11. A region for p-type contact pads    -   12. A region for n-type contact pads    -   13. The arrow indicates a distance between the p-type contact        pad and the n-type contact pad.

FIG. 3 is a top view of another example of an electronic device. Eachnumber represents the following:

-   -   10. A region for drift layer    -   11. A region for p-type contact pads    -   12. A region for n-type contact pads    -   13. The arrow indicates a distance between the p-type contact        pad and the n-type contact pad.

FIG. 4A through 4F depict one example of a fabrication process for anelectronic device using a group III nitride.

In the figure each number represents the following:

-   -   1. Homoepitaxial substrate    -   2. Backside contact    -   3. Drift layer    -   3 b. Etched holes created in the drift layer    -   4. p-type contact pads    -   5. Ohmic contact to the p-type contact pads    -   6. n-type contact pads    -   7. Ohmic contact to the n-type contact pads    -   8. A mask to create etched holes in the drift layer    -   9. A mask to create n-type contact pads.

FIG. 5A through 5F depict one example of a fabrication process for anelectronic device using a group III nitride.

In the figure each number represents the following:

-   -   1. Homoepitaxial substrate    -   2. Backside contact    -   3. Drift layer    -   3 b. Etched holes created in the drift layer    -   3 c. Regrown drift layer    -   4. p-type contact pads    -   5. Ohmic contact to the p-type contact pads    -   6. n-type contact pads    -   7. Ohmic contact to the n-type contact pads    -   8. A mask to create etched holes in the drift layer    -   9. A mask to create n-type contact pads.

FIG. 6 is one example of an electronic device formed using a group IIInitride substrate.

In the figures each number represents the following:

-   -   1. Homoepitaxial substrate    -   3. Drift layer    -   3 a. A depletion region created inside the drift layer    -   4. p-type contact pads    -   5. Ohmic contact to the p-type contact pads    -   6. n-type contact pads    -   7. Ohmic contact to the n-type contact pads    -   10. Drain layer    -   11. Ohmic contact to drain layer.

FIG. 7A through 7F depict one example of a fabrication process for anelectronic device using a group III nitride.

In the figure each number represents the following:

-   -   1. Homoepitaxial substrate    -   3. Drift layer    -   3 b. Etched holes created in the drift layer    -   3 c. Regrown drift layer    -   4. p-type contact pads    -   5. Ohmic contact to the p-type contact pads    -   6. n-type contact pads    -   7. Ohmic contact to the n-type contact pads    -   8. A mask to create etched holes in the drift layer    -   9. A mask to create n-type contact pads.    -   10. Drain layer    -   11. Ohmic contact to drain layer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is an example of the electronic device in this invention.

To realize a GaN-based electronic device with a vertical configuration,a homoepitaxial substrate 1, such as GaN, AlN or generallyGa_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1 may be used. The substratepreferably has a dislocation density less than 5×10⁵ cm⁻². Among manychoices, highly conductive n-type GaN substrates are the best choice tominimize series resistance (i.e. on-resistance). Preferably thesubstrate has an electron concentration greater than 5×10¹⁸ cm⁻³ andmore preferably greater than about 2×10¹⁹ cm⁻³. The substrate may or maynot contain sodium or other element that is used as a mineralizer inammonothermal growth.

Currently, majority of commercially available GaN substrates areproduced by a method called hydride vapor phase epitaxy (HVPE). HVPE isa vapor phase method, which has a difficulty in reducing defect densityless than 5×10⁵ cm⁻². Furthermore, the manufacturing process involvesremoval of the substrate after growing a thick (more than 0.1 mm) GaNlayer, which is quite labor intensive and low yield. In addition,obtaining higher electron concentration than about 2×10¹⁹ cm⁻³ is notcommonly available possibly due to limited incorporation of silicon intoGaN. Therefore, although a vertical device in the current invention canbe fabricated using HVPE-made GaN substrates, it is more preferable touse GaN produced by another method.

To obtain low-cost, low-defect, highly conductive GaN substrates ofwhich density of dislocations and/or grain boundaries is less than about5×10⁵ cm⁻², a new method called ammonothermal growth has been developed[refs. 1-6]. The ammonothermal method is one of the bulk growth methodsof group III nitride crystals using supercritical ammonia.

Growth rate of crystals in supercritical ammonia is typically low. Togrow bulk GaN crystals at a practically useful speed for producingsubstrates, a chemical additive called a mineralizer is added to thesupercritical ammonia. A mineralizer is typically an element or acompound of group I elements or group VII elements, such as potassium,sodium, lithium, potassium amide, sodium amide, lithium amide, ammoniumfluoride, ammonium chloride, ammonium bromide, ammonium iodide andgallium iodide. Sometimes more than two kinds of mineralizers are mixedto attain a good growth condition.

Although most of the mineralizers are interchangeable, sodium is themost favorable mineralizer in terms of growth rate, purity and handling.By ammonothermal growth using sodium, GaN substrates having dislocationdensity less than about 5×10⁵ cm⁻² can be produced. Low defect of GaNsubstrate is beneficial to attaining high breakdown voltage.

GaN substrates grown by ammonothermal growth contain high concentrationof oxygen which attains electron concentration of the substrate higherthan about 5×10¹⁸ cm⁻³ or more preferably about 2×10¹⁹ cm⁻³. Thisfeature is desired to minimize series resistance. For theabove-mentioned reasons, the vertical electronic devices of the currentinvention preferably use GaN substrates produced by the ammonothermalmethod. Optionally the substrate can be an alloy of group III nitrideexpressed as Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1).

The homoepitaxial substrate 1 may be formed by growing one or more bulkcrystals of (0≤x1≤1, 0≤y3≤1) using the ammonothermal method. Thecrystals may be grown under acidic, basic, or neutral conditions in ahigh-pressure reactor as known in the art or as described in any of thegenerally-related patent applications listed above.

An electron donor such as oxygen and/or hydrogen is incorporated intothe bulk crystal during ammonothermal growth by introducing enoughoxygen and/or hydrogen into the growth chamber of the high-pressurereactor as nutrient, mineralizer, seed, ammonia, and any other desiredmaterials are placed in the reactor. Oxygen and hydrogen can beintroduced into the chamber from air by evacuating the reactor ofambient air after loading the raw materials but leaving a sufficientamount of air in the reactor to provide the desired level of oxygen andmoisture in the chamber.

Oxygen and hydrogen can also or alternatively be introduced into thereactor chamber in the form of an oxide or hydride of e.g. an elementused in the mineralizer. For instance, sodium and/or potassium may beused as the mineralizer, and often the sodium and/or potassium added tothe reactor has an amount that has oxidized or moistened. Themineralizer may be oxidized sufficiently in e.g. an oxygen-containingenvironment prior to and/or during insertion into the reactor so thatthe mineralizer provides a sufficient amount of oxygen/hydrogen andprovides the specified level of oxygen/hydrogen concentration in thebulk crystal.

The amount of oxygen and/or hydrogen added to the reactor by any of themethods above is sufficient to provide a substrate with an oxygen and/orhydrogen concentration that is preferably greater than about 5×10¹⁸ cm⁻³and more preferably greater than about 2×10¹⁹ cm⁻³.

Although the majority of conventional group III nitride electronicdevices are fabricated using metalorganic chemical vapor deposition(MOCVD), a drift layer grown by MOCVD tends to show lower breakdownvoltage than the theoretical value. We have considered the possiblereasons for this problem and have reasoned that carbon impurity in thedrift layer during device fabrication could cause the decrease of thebreakdown voltage.

In MOCVD, metalorganic precursors such as trimethylgallium contain ahigh amount of carbon, which is incorporated in the grown film.Therefore, the drift layer 3 of the group III nitride electronic devicein the current invention is preferably formed using a growth methodwhich does not utilize a carbon-containing source or precursor so thatthe drift layer contains little, if any, carbon. The growth method ispreferably HVPE. Other methods such as molecular beam epitaxy (MBE) canbe used.

The drift layer 3 may be formed by HVPE on the first side of the waferso that the impurity level and the electron concentration in the driftlayer 3 are low. The first side of the homoepitaxial substrate 1 ispreferably Ga polar c-plane. However, since the drift layer is typicallythick (typically 5 to 15 microns), using the on-axis c-plane substrateoften creates a rough surface (possibly due to three-dimensionalgrowth). To avoid potential three-dimensional growth and achieve asmooth surface after the drift layer growth, the electronic device ofthe current invention preferably uses a substrate having a Ga-polarc-plane major surface with intentional miscut between 0.2 and 0.6 degreealong m-direction of the crystal. The drift layer may be formed ofGa_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1).

The growth conditions of the drift layer can be adjusted so that nodislocations are newly generated at the interface between the substrateand the drift layer. The adjustments may include one or more ofadjusting growth temperature, temperature ramping profile, and timing ofintroducing reaction gas or source, as known by a person of ordinaryskill.

The dislocation density of the Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1,0≤y2≤1) drift layer can therefore be near or at the same level of thatof the substrate (i.e. preferably less than about 5×10⁵ cm⁻³).

In addition, HVPE can provide the drift layer with a lower impurityconcentration than that of the ammonothermal substrate. Therefore, theelectron concentration of the drift layer 3 can preferably be lower thanabout 5×10¹⁶ cm⁻³ or more preferably lower than about 1×10¹⁶ cm⁻³.

The carbon concentration of the drift layer is preferably less thanabout 1×10¹⁶ cm⁻³. The high structural quality and high purity nature ofthe drift layer enables faster electron mobility (i.e. lower seriesresistance) and higher breakdown voltage.

The above-mentioned combination of the layer structure and thecorresponding preferred fabrication methods (i.e. ammonothermal methodfor the substrates and HVPE method for the drift layer) for thestructure are derived from the design concept of the electronic powerdevices in the current invention. However, these conditions are a partof other design factors.

To realize high-performance high-power devices with normally-offoperation, low-series resistance, high-breakdown voltage, fast switchingspeed, high efficiency and low cost, the device preferably alsoincorporates the following components.

A contact 2 (preferably Ohmic) can be created on the backside of thesubstrate (i.e. opposite side to the drift layer, as illustrated in FIG.1). The backside contact can be made after fabricating p-type contactpads 4 and optionally n-type contact pads 6. A backside Ohmic contact 2is typically Ti/Al and is fabricated by using e.g. conventional electronbeam evaporators, resistive heater evaporators, sputtering, laserdepositions or other known metallization equipment and methods.

At least one p-type contact pad of Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1,0≤y3≤1) (feature 4 of FIG. 1) is fabricated on the piece having thesubstrate, drift layer, and backside contact to make a diode, and atleast one set of pads (at least one p-type contact pad ofGa_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) (feature 4) and at leastone n-type contact pad of Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1)(feature 6)) are required to make a transistor.

In conventional vertical type electronic devices of group III nitride,p-type contact pads are typically formed by selective etching of thedrift layer followed by selective MOCVD growth using a silicon dioxidemask. However, the fabricated electronic devices in the conventionalmethods tend to show high level of leakage current as well as lowbreakdown voltage under reverse bias.

We considered the possible reasons for this problem and reasoned thathigh concentration of hydrogen in the p-type contact pads is a possiblecause. Since MOCVD growth uses ammonia as well as hydrogen carriergases, the group III nitride film grown by MOCVD contains high level ofhydrogen (higher than 1×10¹⁷ cm⁻³). Also, ammonia and/or hydrogen inMOCVD environment may etch the silicon dioxide mask to emit Si and/or Ointo the growth environment, which in turn will be incorporated into thep-type contact pads.

Therefore, in one embodiment of the current invention, the p-typecontact pads of Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) 4 areformed by a method which does not use ammonia or hydrogen as a carriergas so that the p-type contact pads contain little hydrogen. As aresult, the hydrogen concentration of the p-type contact pads 4 ispreferably less than one hundredth of that in the substrate and lessthan one tenth of that in the drift layer. The low hydrogen feature ofthe p-type contact pads may consequently help avoid leakage current andbreakdown of the transistor.

Another possible reason for the low breakdown voltage is that MOCVDregrowth process using a silicon dioxide mask causes incorporation of Siand/or O into the p-type contact pads probably due to high growthtemperature (approximately 1050° C.). Therefore in one embodiment of thecurrent invention, the p-type contact pads ofGa_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) 4 are formed at or below800° C. to avoid incorporating Si and/or O into the pads. With lowtemperature deposition, the silicon concentration in the p-type contactpads are preferably suppressed at or below 1×10¹⁸ cm⁻³.

The p-type contact pads may be formed using e.g. pulsed laser deposition(PLD) or molecular beam epitaxy (MBE). PLD provides high selectivityagainst a mask used to define features (e.g. silicon dioxide mask), andPLD also provides high coverage on the etched trenches.

Additionally, the hole concentration in the p-type contact pads can behigher than about 1×10¹⁷ cm⁻³ without p-type activation annealing in thepresent method. PLD or MBE can consequently be used in the preferredmethod without the step of annealing p-type material to provide adesired hole concentration.

p-type contact pads having both low hydrogen concentration and high holeconcentration help provide an electronic device with high breakdownvoltage and high efficiency.

The impurity to obtain p-type conduction may preferably be Mg althoughBe can also be used. The Mg may be supplied separately during depositionprocess or may be pre-mixed in the group III source.

Similarly, the n-type contact pads of Ga_(1-x4-y4)Al_(x4)In_(y4)N(0≤x4≤1, 0≤y4≤1) 6 may be formed by PLD or MBE. The impurity to obtainn-type conduction may preferably be Ge although Si can also be used. Thegermanium concentration of the n-type contact pads 6 is preferably morethan about 1×10¹⁸ cm⁻³ and hydrogen concentration of the n-type contactpad is preferably less than about 7×10¹⁶ cm⁻³. The electronconcentration of the n-type contact pads 6 is preferably higher thanabout 1×10¹⁸ cm⁻³. With these parameters, electrons are efficientlyinjected to the drift layer under transistor operation.

If the electronic device has p-type contact pads 4 as depicted in FIG.1, a depletion region 3 a is formed inside drift layer 3. Free electronsand holes do not exist inside depletion region 3 a. The spacing betweenthe p-type contact pads may be selected so that the depletion regions ofthe adjacent p-type contact pads touch together.

Alternatively, the device structure in FIG. 1 can be fabricated by thefollowing steps. The drift layer 3 and the p-type contact pad 4 areformed by MOCVD in one growth run. In this case, there is littleaccumulation of silicon at the interface between the drift layer 3 andthe p-type contact pad 4. Consequently, in this method, a portion of thedrift layer is deposited on the homoepitaxial substrate. Next, a firstlayer of p-type contact pad material is deposited on the drift layer.These two layers may be formed in the same epitaxial depositionequipment without moving the device from one reactor to another. Theequipment may be MOCVD equipment, for instance. Then, the layer ofp-type contact pad material parts is etched to the first drift layer 3to form p-type contact pads 4. Using a selective growth of MOCVD, HVPE,MBE or other growth method, the etched areas are filled with a secondlayer of drift layer material to the tops of the p-type contact pads,followed by formation of n-type contact pads 6. After growing allnecessary layers and portions, Ohmic contacts are formed on p-typecontact pads 4, n-type contact pads 6 and the other side of thesubstrate 1. The p-contact pads have a height, given the carrierconcentrations selected for the drift layer and the p-type contact padmaterial, that assures that the depletion region extends through thesecond drift layer and into the first drift layer as is discussed inmore detail in Example 4.

The n-type contact pads are surrounded by the p-type contact pads whenviewing the device from above the top surface as in e.g. FIG. 2 or FIG.3.

FIG.2 shows one example layout of the contact pads. The n-type contactpads 13 are surrounded by the p-type contacts pads 12. The separationwidth w (indicated as 14) may be set narrow enough so that the depletionregions from p-type contact pads extend under the n-type contact pads.

FIG.3 shows another example layout of the contact pads, where the deviceshape is hexagonal and n-type contact pads are also hexagonal. Thislayout may ensure isotropic electronic characteristics by aligning thepad shape to the crystallographic orientation such as m-plane ora-plane.

In either case, the separation width is preferably set narrow enough sothat the depletion regions created by the p-type contact pad entirelycover the area under the n-type contact pads. This feature is preferablewhen making normally-off devices.

Both p-type contact pads 4 and n-type contact pads 6 are on the sameside of the drift layer. The p-type contact pads 4 and n-type contactpads 6 may therefore be separate and do not directly touch one another.

The p-type contact pads 4 and n-type contact pads 6 may also beseparated from one another vertically, as illustrated in FIG. 1. Thep-type contact pads 4 are preferably formed closer to the substratesurface than the n-type contact pads so that carrier path can becontrolled easily. As explained below, this can be realized by selectiveetching followed by deposition method such as PLD.

One highly preferable feature of the electronic device of the currentinvention is that the bandgap of the p-type contact pads 4 may be largerthan the bandgap of the drift layer 3. When the forward bias is appliedbetween the p-type contact pads 4 (gate) and the n-type contact pads 6(source), electrons are injected from the n-type contact pads 6 to thedrift layer 3 and holes are injected from the p-type pad 4 to the driftlayer 3. The electronic device may have two distinct properties bysetting the bandgap of the p-type contact pads 4 larger than the bandgapof the drift layer 3: (1) the electrons in the drift layer cannot beinjected into the p-type contact pads 4, and (2) the hole injectioncurrent from the p-type contact pad 4 to the drift layer 3 may becomemore than about 10,000 times higher than the electron injection currentfrom the drift layer 3.

For example, when the bandgap of the p-type contact pads 4 is largerthan the bandgap of the drift layer 3 by approximately 0.3 eV, the holeinjection current from the p-type contact pad 4 to the drift layer 3 maybecome more than about 100,000 times higher than the electron injectioncurrent from the drift layer 3 to the p-type contact pad 4. If thebandgap difference is 0.25 eV, the hole injection current from thep-type contact pad 4 to the drift layer 3 may become more than about15,000 times higher than the electron injection current from the driftlayer 3 to the p-type contact pad 4. This produces highly-efficienttransistors.

The bandgap of the drift layer and pad can be easily controlled byadjusting alloy compositions of the group III nitride. For example, theGaN may be used for the substrate 2, the drift layer 3, and n-typecontact pads 6 and Ga_(0.89)Al_(0.11)N may be used for the p-typecontact pads 4. In this case, the difference in the bandgap between thep-type contact pad and the drift layer becomes about 0.3 eV.Alternatively Ga_(0.91)Al_(0.09)N may be used for p-type contact. Inthis case the difference in the bandgap between the p-type contact padand the drift layer becomes about 0.25 eV. The relationship between thebandgap and alloy composition are well known to the person of theordinary skill. Consequently, the person of ordinary skill can provideother devices with a desired difference in bandgap using the guidanceabove and their background knowledge and experience.

An Ohmic contact to the p-type contact pads 5 and an Ohmic contact tothe n-type contact pads 7 can each be formed by a conventionalmetallization method similar to the metallization method used to formbackside Ohmic contact 2. The Ohmic contact 5 to the p-type contact padsmay be Ni/Au, and the Ohmic contact 7 to the n-type contact pads 6 canbe Ti/Al.

FIG. 4 A-F show one example of a process flow to fabricate an electronicdevice of the current invention. First a substrate ofGa_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1) 1 is obtained or preparedpreferably by ammonothermal growth. The dislocation density of thesubstrate is preferably less than 5×10⁵ cm⁻³ and the electronconcentration of the substrate is preferably higher than about 5×10¹⁸cm⁻³ or more preferably 2×10¹⁹ cm⁻³. The surface of the substrate ispreferably oriented to c-plane with miscut angle between 0.2 to 0.6degree along m-direction.

Then, a drift layer 3 of Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1) isgrown on the Ga polar surface of the substrate 1 preferably by HVPE(FIG. 4A). The thickness is about 5 to 20 microns or more preferably 7to 15 microns. The carrier concentration is preferably controlled to beless than about 5×10¹⁶ cm⁻³ or more preferably about 1×10¹⁶ cm⁻³ toensure normally-off operation. In addition, the carbon concentration inthe drift layer is preferably controlled to be less than about 1×10¹⁶cm⁻³.

To make trenches on the surface of the drift layer, a mask 8 (FIG. 4B)of a material such as silicon dioxide, silicon nitride, other dielectricor metal material is formed using a conventional deposition method,followed by photolithography. Then, using an etching technique such asreactive ion etching or wet-etching, trenches 3 b (FIG. 2B) for p-typecontact pads are formed. The etching process preferably provides asmooth sidewall and rounded bottom to avoid concentration of electricfield at sharp corners during device use.

After this trench formation, p-type contact pads ofGa_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) (feature 4 of FIG. 4C) areformed by a deposition method which does not use a hydrogen containingsource. One can use the same residual mask 8 of FIG. 4B to achieveselective deposition, or one can re-deposit a new mask for thedeposition. The deposition method is preferably PLD and the depositiontemperature is preferably less than 800° C. Mg or Be may be used toobtain p-type conduction with hole concentration preferably higher thanabout 1×10¹⁷ cm⁻³. The Mg or Be concentration is preferably higher thanabout 1×10²⁰ cm⁻³. The hydrogen concentration in the p-type contact padsis preferably controlled to be less than one hundredth of that of thesubstrate 1 and one tenth of that of the drift layer 3 as explainedabove. Silicon concentration is preferably controlled to be at or below1×10¹⁸ cm⁻³.

If dry etching is used to form trenches, the etching reaction chamber ispreferably connected to the deposition chamber of the p-type contactpads so that the devices can be transferred without exposing them to theair.

The amount of Al, Ga, and In may be adjusted so that the bandgap of thep-type contact pads becomes larger than that of the drift layer. In thePLD, the group III sources (i.e. Al, Ga, In) can be mixed together orcan be separate. Also, Mg or Be can be mixed to the group III sources orcan be separate. After the formation process of the p-type contact pads,the masks 8 are removed from the surface (FIG. 4C).

To form n-type contact pads, another mask 9 (FIG. 4D) is formed on thesurface in the similar way as the mask 8 of FIG. 4B. Then, n-typecontact pads are formed by a deposition method which does not usehydrogen containing source. Similar to the p-type contact pads, thedeposition method is preferably PLD with Ge or Si as n-type dopant. Theelectron concentration of the n-type contact pad is preferablycontrolled to be higher than about 1×10¹⁸ cm⁻³, the germaniumconcentration is preferably controlled to be higher than about 1×10¹⁸cm⁻³, and the hydrogen concentration is less than about 7×10¹⁶ cm⁻³.After formation of the n-type contact pads, the mask 9 is removed(FIG.2E)

Finally, FIG. 4F depicts Ohmic contacts 5 on the p-type contact pads 4,Ohmic contacts 7 on the n-type contact pads 6, and Ohmic contact on thebackside of substrate 2. The Ohmic contacts may be formed by aconventional metallization method such as e-beam evaporation, thermalevaporation, sputtering or other deposition methods. In this processflow, transistors with source (n-type contact pads), gate (p-typecontact pads) and drain (substrate) are fabricated.

FIG. 6 is an example of an electronic device of this invention for highfrequency applications. Instead of having a conductive substrate, thedevice in FIG. 6 is fabricated on a semi-insulating substrate 1. GaNsubstrates grown by ammonothermal growth can be made semi-insulating bydoping an appropriate amount of one or more impurities such as Mg, Ca,Zn, Bi, Fe, Mo, etc. such as the substrate disclosed in U.S. Pat. Nos.9,255,342 and 9,435,051 mentioned and incorporated by reference above. Asemi-insulating substrate can reduce high-frequency leakage current byminimizing parasitic capacitance of the device. For this reason, thevertical electronic devices of the current invention preferably use GaNsubstrates produced by the ammonothermal method. Optionally thesubstrate can be an alloy of group III nitride expressed asGa_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1). The resistivity of thesubstrate is preferably at least 1 kΩ cm. In this case a drain layermade of n-type Ga_(1-x5-y5)Al_(x5)In_(y5)N (0≤x5≤1, 0≤y5≤1) and contactto drain layer is added as shown in FIG. 6.

The following examples supplement the discussion above and are providedto illustrate certain specific embodiments of the invention. Theexamples therefore provide the person with ordinary skill with guidanceon how to carry out the invention described above.

EXAMPLE 1

A bulk crystal of GaN was grown with the basic ammonothermal method in apressure reactor having internal volume of 127 cc using polycrystallineGaN (15 g) as a nutrient, supercritical ammonia (53% fill to the reactorvolume) as a solvent, and sodium (5 mol % to ammonia) as a mineralizer.The growth temperature was between 500 to 600° C., and growth extendedto 181 days. A bulk crystal of GaN was grown on a c-plane GaN seedcrystal. The bulk crystal was approximately 10 mm² thick. Then thecrystal was sliced into wafers using a multiple wire saw. Nine wafersapproximately 1 mm thick each were sliced out of one bulk GaN crystal.These wafers were ground to make c-plane miscut to be about 0.4°. Then,they were lapped with diamond slurry and polished using CMP. The defectdensity of one of these wafers was characterized with X-ray topography.The dislocation density was about 4×10⁴ cm⁻². The electron concentrationwas about 2.5×10¹⁹ cm⁻³ due to incorporation of oxygen and/or hydrogenintroduced into the reactor from air retained in the reactor as it wasprepared for growth, from oxygen and/or hydrogen deliberately added tothe reactor, and/or from oxygen and/or hydrogen that accompaniedmineralizer.

EXAMPLE 2

Using GaN wafers prepared by the ammonothermal growth in the Example 2,GaN drift layers were grown by HVPE. In each run, one wafer ofapproximately 10 mm×10 mm (L×W) in size was used. Inside the HVPEreactor, hydrogen chloride gas was passed over heated Ga and then mixedwith ammonia prior to encountering the heated wafer. The temperature ofthe Ga was in the range of 800 to 1000° C., and the temperature of thewafer was in the range of 900 to 1150° C. In this example, GaN having athickness of about 10 microns was grown on the Ga-polar surface ofammonothermal c-plane GaN wafers. The growth rate was in the range of 50to 400 microns per hour. Using Ti/Al for the backside cathode contactand Ni/Au for the front side anode contact, I-V characteristics of thedrift layer were measured. The I-V characteristics did not showconductance of current for both forward and reverse bias direction.Measuring C-V characteristic showed carrier concentration was less thanabout 1×10¹⁶ cm⁻³. The carbon concentration in the layer evaluated bysecondary mass spectroscopy (SIMS) was about 6×10¹⁵ cm⁻³.

EXAMPLE 3

Transistors may be formed using the GaN drift layer fabricated on thesubstrate of Example 2. First, a SiO₂ layer of about 2 microns isdeposited with plasma CVD using SiH₄ gas and oxygen gas. Using aconventional photolithography technique, a mask pattern is created andthe drift layer is etched to make trenches by ICP plasma etching usingCl₂ gas. The trench width is about 10 microns and trench depth is about1 microns. Without exposing the etched wafer to air, the wafer istransferred to PLD chamber to form p-type contact pads. Using the samepatterned SiO₂ mask, AlGaN p-type contact pads with Mg doping aredeposited by PLD at 600° C. using premixed Ga, Al and Mg melt. Thethickness of the p-type contact pads is about 1 microns, and the lateralseparations between p-type contact pads are about 2 microns. The widthof the pad is about 10 microns. The Mg concentration is about 1×10²⁰cm⁻³ and hole concentration is about 1×10¹⁷ cm⁻³. Since hydrogen isminimized in the reaction ambient, the hydrogen concentration in thep-type contact pad is below the detection limit of SIMS (less than7×10¹⁶ cm⁻³). The aluminum mole fraction in AlGaN is about 11%, whichmakes the bandgap of the AlGaN p-type contact pads larger than that ofthe GaN drift layer by about 0.3 eV. The hole injection current from thep-type contact pads to the drift layer is calculated to be about 100,000times higher than the electron injection current from the drift layer tothe p-type contact pads.

Then another layer of SiO₂ mask is deposited and patterned to formn-type contact pads. Since the photolithography process is conductedoutside of vacuum system, the substrate is exposed to air. Afterpatterning of the SiO₂ mask, the substrate is loaded to a plasmacleaning chamber, and the top surface of the drift layer is cleaned byetching a small amount from the layer's top. The removed thickness isless than about 5 nm in this example. Then, the substrate is transferredto the PLD deposition system to form n-type contact pads of GaN usingpremixed Ga and Ge. The n-type contact pads are formed right in themiddle of the p-type contact pads. The pad width is about 1 microns andthe thickness is about 0.5 microns. The Ge concentration is about 5×10¹⁹cm⁻³ and the electron concentration is about 1×10¹⁹ cm⁻³. Using the samemask, Ti/Al Ohmic contacts are formed on top of the n-type contact pads.

Finally, Ni/Au p-type contact pads are formed using lift-off process ofphoto resist and backside Ti/Al contact is formed.

The transistor withstands source-drain voltage of about 1500 V, and theseries resistance under forward bias between source and gate is about 1mΩ cm.

EXAMPLE 4

Transistors may be formed using the GaN substrates in Example 1. Thecarrier concentration of the GaN substrate is 2×10¹⁹ cm⁻³. A first driftlayer of undoped GaN having a thickness of 10 microns is fabricated onthe substrate by MOCVD followed by successive growth of a p-GaN layerhaving a thickness of 0.5 microns by MOCVD in the same epitaxialdeposition device (FIG. 5A). The carrier concentration of the driftlayer is less than 1×10¹⁵ cm⁻³ and the carrier (hole) concentration ofthe p-GaN layer is 1×10¹⁸ cm⁻³. With these carrier concentrations in thedrift layer and p-GaN, a depletion region extends three dimensionallyabout 1.4 microns into the drift layer after completion of fabricationas shown in FIG. 5F. In other words, a depletion region in the verticaldevice extends in a horizontal direction by 1.4 microns from the edge ofits p-type contact pad as well as toward the substrate from theinterface of the p-type contact pad by 1.4 microns.

Then, parts of the p-GaN is etched down to the drift layer using aconventional photolithography and dry etching such as reactive ionetching (ME) to form p-type contact pads (FIG. 5B). Using SiO₂ mask ormetal mask, a second drift layer of undoped GaN is selectively grown inthe etched regions and on the first drift layer by MOCVD (FIG. 5C),followed by mask patterning (FIG. 5D) and growth of n-GaN for n-typecontact pads (FIG. 5E). The carrier concentration of the n-GaN is 5×10¹⁸cm⁻³. By forming Ohmic contacts to the n-type contact pad, p-typecontact pad and the backside of the substrate, a transistor can beformed (FIG. 5F).

The lateral size of the etched region is about 2 microns. Since thedepletion region extends to 1.4 microns, the depletion region in theregrown undoped GaN completely fill the space, preventing current flowunder zero bias condition between the p-type contact pad and the n-typecontact pad. This enables normally-off operation of the transistor.

EXAMPLE 5

A transistor may be formed using a semi-insulating GaN substrate. FIG. 7is an example process flow for making a transistor. The resistivity ofthe GaN substrate 1 is 1 kΩ cm. A first drain layer 10 of n-GaN having athickness of 0.2 microns and a drift layer 3 of undoped GaN having athickness of 0.1 microns is fabricated on the substrate by MOCVDfollowed by successive growth of a p-GaN layer 4 having a thickness of0.2 microns by MOCVD, with all of the GaN layers being depositedsequentially in the same epitaxial deposition device without removingthe substrate (FIG. 7A). The carrier concentration of the drain layer 10is 5×10¹⁸ cm⁻³, drift layer 3 is less than 1×10¹⁵ cm⁻³ and the carrier(hole) concentration of the p-GaN layer 4 is 1×10¹⁸ cm⁻³. With thesecarrier concentrations in the drift layer and p-GaN, a depletion regionextends three dimensionally about 1.4 microns into the drift layer aftercompletion of fabrication as shown in FIG. 7F. In other words, adepletion region in the vertical device extends in a horizontaldirection by 1.4 microns from the edge of its p-type contact pad as wellas toward the substrate from the interface of the p-type contact pad by1.4 microns.

After depositing the p-GaN layer 4, mask 8 is formed using conventionalphotolithography, and regions 3 b of the p-GaN are etched down to thedrift layer using conventional dry etching such as reactive ion etching(RIE) to form p-type contact pads 4 of FIG. 7B. Using a SiO₂ mask ormetal mask, a second drift layer 3 e of undoped GaN is selectively grownin the etched regions and on the first drift layer by MOCVD as shown inFIG. 7C, followed by mask patterning to form mask 9 as shown in FIG. 7D.n-GaN for n-type contact pads 6 as shown in FIG. 7E. The carrierconcentration of the n-GaN 6 is 5×10¹⁸ cm⁻³. Then, Ohmic contacts 7, 5,and 11 to the n-type contact pad 6, the p-type contact pad 4, and thedrain layer 10, respectively, are formed. The Ohmic contact to the drainlayer is formed by etching the layers down to the drain layer anddepositing Ohmic contact 11, as shown by FIG. 7F.

The lateral size of the etched region is about 2 microns. Since thedepletion region extends to 1.4 microns, the depletion region in theregrown undoped GaN completely fills the space beneath the n-typecontact pads and, in this case, the p-type contact pads as well,preventing current flow under zero bias condition between the p-typecontact pad and the n-type contact pad. This enables normally-offoperation of the transistor.

The following is therefore disclosed by way of example and not by way oflimitation in view of the discussion above:

-   -   1. A vertical electronic device comprising an n-type contact        pad, a p-type contact pad, a drift layer having a thickness, a        drain, and a substrate, wherein the drift layer is positioned        between the n-type contact pad and the drain so that an        electrical current flows from the n-type contact pad through the        thickness of the drift layer and to the drain, the n-type        contact pad being adjacent to the p-type contact pad, and        wherein the n-type contact pad and the p-type contact pad are        positioned sufficiently closely to one another on the substrate        so that a depletion region in the drift layer created by the        p-type contact pad prevents current flow from the n-type contact        pad on the drift layer to the Ohmic contact when no voltage is        applied between the n-type contact pad and the p-type contact        pad.    -   2. An electronic device of paragraph 1, wherein the substrate        has a first side and a second side opposite the first side, and        wherein the n-type contact pad, the p-type contact pad, the        drift layer, and the drain reside on the first side of the        substrate.    -   3. An electronic device of paragraph 1, wherein the substrate        has a first side and a second side opposite the first side, and        wherein the n-type contact pad, the p-type contact pad, and the        drift layer reside on the first side of the substrate and        wherein the drain resides on the second side of the substrate.    -   4. An electronic device of any of paragraphs 1-3, wherein the        p-type contact pad has a bandgap that is larger than a bandgap        of the drift layer.    -   5. An electronic device of any of paragraphs 1-4, wherein the        p-type contact pad comprises Ga_(1-x3-y3)Al_(x3)In_(y3)N        (0≤x3≤1, 0≤y3≤1), the n-type contact pad comprises        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1), the drift layer        comprises Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1), and        wherein        -   (a)the substrate has a dislocation density less than 5×10⁵            cm⁻²;        -   (b)the substrate has an electron concentration higher than            5×10¹⁸ cm⁻³;        -   (c)the drift layer has an electron concentration lower than            5×10¹⁶ cm⁻³; and        -   (d)the p-type contact pad has a hole concentration higher            than 1×10¹⁷ cm⁻³.    -   6. An electronic device of paragraph 4 or paragraph 5 and        further comprising an Ohmic contact on the drain, a drift layer        of Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1), an Ohmic        contact attached to the p-type contact pad, and at least one        n-type contact pad of Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1,        0≤y4≤1) on the drift layer spaced apart from the p-type contact        pad.    -   7. An electronic device comprising a substrate, an Ohmic contact        on one side of the substrate, a drift layer of        Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1) on an opposite side        of the substrate, at least one p-type contact pad of        Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) attached to the        drift layer, an Ohmic contact attached to the p-type contact        pad, and at least one n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer        spaced apart from the p-type contact pad, wherein:        -   (a)the substrate has a dislocation density less than 5×10⁵            cm⁻²;        -   (b)the substrate has an electron concentration higher than            5×10¹⁸ cm⁻³;        -   (c)the drift layer has an electron concentration lower than            5×10¹⁶ cm⁻³; and        -   (d)the p-type contact pad has a hole concentration higher            than 1×10¹⁷ cm⁻³.    -   8. An electronic device of any of paragraphs 1-7, wherein the        p-type contact pad and the n-type contact pad are positioned        sufficiently close to one another on the substrate so that a        depletion region in the drift layer created by the p-type        contact pad prevents current flow from the n-type contact pad on        the drift layer to the Ohmic contact when no voltage is applied        between the n-type contact pad and the p-type contact pad.    -   9. An electronic device of any of paragraphs 1-8, wherein the        p-type contact pad is located closer to the substrate than the        n-type contact pad.    -   10. An electronic device of any of paragraphs 1-9, wherein the        n-type contact pad is surrounded by the p-type contact pad.    -   11. An electronic device of paragraph 10, wherein all sides of        contact pads are aligned to m-plane of the drift layer.    -   12. An electronic device of any of paragraphs 1-11, wherein the        electron concentration of the n-type contact pad is more than        1×10¹⁸ cm⁻³.    -   13. An electronic device of any of paragraphs 1-12, wherein the        substrate is Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1).    -   14. An electronic device comprising a substrate of        Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1), an Ohmic contact        on one side of the substrate, a drift layer of        Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1) on an opposite side        of the substrate, at least one p-type contact pad of        Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) attached to the        drift layer, and an Ohmic contact attached to the p-type contact        pad, wherein:        -   (a)the substrate has a dislocation density less than 5×10⁵            cm⁻²;        -   (b)the substrate has an electron concentration higher than            5×10¹⁸ cm⁻³;        -   (c)the drift layer has an electron concentration lower than            5×10¹⁶ cm⁻³;        -   (d)the p-type contact pad has a hole concentration higher            than 1×10¹⁷ cm⁻³, and        -   (e)the p-type contact pad has a bandgap that is larger than            a bandgap of the drift layer.    -   15. An electronic device of paragraph 14, wherein the p-type        contact pad's bandgap is sufficiently greater than the drift        layer's bandgap to provide the electronic device with a hole        injection current from the p-type contact pad to the drift layer        that is more than 15,000 times higher than an electron injection        current from the drift layer to the p-type contact pad.    -   16. An electronic device of paragraph 14 or paragraph 15 further        comprising at least one n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer        that is spaced a sufficient distance from the p-type contact pad        that the n-type contact pad is not directly touching the p-type        contact pad.    -   17. An electronic device of paragraph 16, wherein the p-type        contact pad is located close to the substrate than the n-type        contact pad.    -   18. An electronic device of paragraph 16 or paragraph 17,        wherein the n-type contact pad is surrounded by the p-type        contact pad.    -   19. An electronic device of paragraph 18, wherein all sides of        contact pads are aligned to m-plane of the drift layer.    -   20. An electronic device of any of paragraph 16 through        paragraph 19, wherein the electron concentration of the n-type        contact pad is more than 1×10¹⁸ cm⁻³.    -   21. An electronic device of any of paragraph 16 through        paragraph 20, wherein the germanium concentration of the n-type        contact pad is more than 1×10¹⁸ cm⁹⁻³ and hydrogen concentration        of the n-type contact pad is less than 7×10¹⁶ cm⁻³.    -   22. An electronic device of any of paragraph 16 through        paragraph 21, wherein the electron concentration of the drift        layer is low enough to prevent electric current flowing from the        Ohmic contact on one side of the substrate to the n-type contact        pad when no voltage is applied between the n-type contact pad        and the p-type contact pad.    -   23. An electronic device of any of paragraph 14 through        paragraph 22 further comprising p-GaN layer between the p-type        contact pad and the Ohmic contact to the p-type contact pad.    -   24. An electronic device of any of paragraph 14 through        paragraph 23, wherein the p-type contact pad and the n-type        contact pad are positioned sufficiently close to one another on        the substrate so that a depletion region in the drift layer        created by the p-type contact pad prevents current flow from the        n-type contact pad on the drift layer to the Ohmic contact when        no voltage is applied between the n-type contact pad and the        p-type contact pad.    -   25. An electronic device comprising a substrate of        Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1), a drain layer of        Ga_(1-x5-y5)Al_(x5)In_(y5)N (0≤x5≤1, 0≤y5≤1) and a drift layer        of Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1) on one side of        the substrate, at least one p-type contact pad of        Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) attached to the        drift layer, an Ohmic contact attached to the p-type contact        pad, and at least one n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer        spaced apart from the p-type contact pad, wherein:        -   (a)the substrate has a dislocation density less than 5×10⁵            cm⁻²;        -   (b)the substrate has a resistivity of at least 1 kΩ cm;        -   (c)the drift layer has an electron concentration lower than            5×10¹⁶ cm⁻³; and        -   (d)the p-type contact pad has a hole concentration higher            than 1×10¹⁷ cm⁻³.    -   26. An electronic device of paragraph 25, wherein the p-type        contact pad is located closer to the substrate than the n-type        contact pad.    -   27. An electronic device of paragraph 25 or paragraph 26,        wherein the n-type contact pad is surrounded by the p-type        contact pad.    -   28. An electronic device of paragraph 27, wherein all sides of        contact pads are aligned to m-plane of the drift layer.    -   29. An electronic device of any of paragraphs 25-28, wherein the        p-type contact pad has a bandgap that is larger than a bandgap        of the drift layer.    -   30. An electronic device of any of paragraphs 25-29 and further        comprising a drain layer between the substrate and the drift        layer.    -   31. An electronic device of paragraph 30, wherein the drain        layer extends past the drift layer, and the drain layer has an        Ohmic contact on the same side of the substrate with the p-type        contact pad and the n-type contact pad.    -   32. An electronic device of paragraph 31, wherein the p-type        contact pad and the n-type contact pad are positioned        sufficiently close to one another on the substrate that a        depletion region in the drift layer created by the p-type        contact pad prevents current flow from the n-type contact pad on        the drift layer to the drain layer's Ohmic contact when no        voltage is applied between the n-type contact pad and the p-type        contact pad.    -   33. An electronic device comprising a substrate of        Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1), an Ohmic contact        on one side of the substrate, a drift layer of        Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1) on an opposite side        of the substrate, at least one p-type contact pad of        Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) attached to the        drift layer, and an Ohmic contact connected to the p-type        contact pad, wherein:        -   (a) the substrate has a dislocation density less than 5×10⁵            cm⁻²;        -   (b) the substrate has an electron concentration higher than            5×10¹⁸ cm⁻³;        -   (c) the drift layer has an electron concentration lower than            5×10¹⁶ cm⁻³;        -   (d)the p-type contact pad has a hole concentration higher            than 1×10¹⁷ cm⁻³, and        -   (e) the p-type contact pad has a hydrogen concentration less            than one hundredth of a hydrogen concentration of the            substrate.    -   34. An electronic device of paragraph 33, wherein the hydrogen        concentration of the p-type contact pad is less than one tenth        of a hydrogen concentration of the drift layer.    -   35. An electronic device of paragraph 33 or paragraph 34,        wherein the hydrogen concentration of the substrate is more than        5×10¹⁸ cm⁻³.    -   36. An electronic device of any of paragraphs 33 through 35,        wherein an oxygen concentration of the p-type contact pad is        less than one hundredth of an oxygen concentration of the        substrate.    -   37. An electronic device of any of paragraphs 33 through 36,        wherein a silicon concentration of the p-type contact pad is        less than 1×10¹⁸ cm⁻³.    -   38. An electronic device of any of paragraphs 33 through 37,        wherein the oxygen concentration of the substrate is more than        5×10¹⁸ cm⁻³.    -   39. An electronic device of any of paragraphs 33 through 38,        wherein both oxygen concentration and hydrogen concentration of        the substrate are more than 5×10¹⁸ cm⁻³.    -   40. An electronic device of any of paragraphs 33 through 39,        wherein a carbon concentration of the drift layer is less than        1×10¹⁶ cm⁻³.    -   41. An electronic device of any of paragraph 33 through        paragraph 40, wherein said opposite side of the substrate is        physically inclined from gallium polar c-plane by about 0.2 to        about 0.6 degrees.    -   42. An electronic device of any of paragraph 33 through        paragraph 41, wherein a bandgap of the p-type contact layer is        larger than a bandgap of the drift layer.    -   43. An electronic device of paragraph 42, wherein the p-type        contact pad's bandgap is sufficiently greater than the drift        layer's bandgap to provide the electronic device with a hole        injection current from the p-type contact pad to the drift layer        that is more than 15,000 times higher than an electron injection        current from the drift layer to the p-type contact pad.    -   44. An electronic device of any of paragraph 33 through        paragraph 43 further comprising an n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer        positioned away from the p-type contact pad so that the n-type        contact pad does not directly touch the p-type contact pad.    -   45. An electronic device of paragraph 44, wherein the p-type        contact pad is located closer to the substrate than the n-type        contact pad.    -   46. An electronic device of paragraph 44 or paragraph 45,        wherein the n-type contact pad is surrounded by the p-type        contact pad.    -   47. An electronic device of any of paragraph 44 through        paragraph 46, wherein the electron concentration of the n-type        contact pad is more than 1×10¹⁸ cm⁻³.    -   48. An electronic device of any of paragraph 44 through        paragraph 47, wherein a germanium concentration of the n-type        contact pad is more than 1×10¹⁸ cm⁻³ and a hydrogen        concentration of the n-type contact pad is less than 7×10¹⁶        cm⁻³.    -   49. An electronic device of any of paragraph 44 through        paragraph 48, wherein the electron concentration of the drift        layer is low enough to prevent electric current flow from the        Ohmic contact to the n-type contact pad when no voltage is        applied between the n-type contact pad and the p-type contact        pad.    -   50. An electronic device of paragraph 49, wherein the electron        concentration of the drift layer is less than 1×10¹⁶ cm⁻³.    -   51. An electronic device of any of paragraph 1 through paragraph        50, wherein the substrate is made by an ammonothermal method and        the electron concentration is higher than 2×10¹⁹cm⁻³.    -   52. A method of fabricating an electronic device comprising:        -   (a) growing a drift layer of Ga_(1-x2-y2)Al_(x2)In_(y2)N            (0≤x2≤1, 0≤y2≤1) by vapor phase epitaxy on a first side of a            substrate of Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1);        -   (b)forming a p-type contact pad of            Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) on the drift            layer by a deposition method which does not use a            hydrogen-containing source;        -   (c) forming an Ohmic contact on a second side of the            substrate.    -   53. A method of fabricating an electronic device of paragraph        52, wherein the step of growing the drift layer by vapor phase        epitaxy does not use a carbon-containing source.    -   54. A method of fabricating an electronic device of paragraph 52        or paragraph 53 further comprising forming n-type contact pads        of Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift        layer by a deposition method which does not use a        hydrogen-containing source, and wherein the n-type contact pads        are separated from the p-type contact pads by a sufficient        distance that the n-type contact pads do not directly touch the        p-type contact pads.    -   55. A method of fabricating an electronic device of paragraph 54        further comprising etching a trench in the drift layer to make        the p-type contact pad closer to the substrate than the n-type        contact pads are to the substrate.    -   56. A method of fabricating an electronic device of paragraph        55, wherein the p-type contact pad is formed without exposing        the device to air after etching the trench in the drift layer.    -   57. A method according to any of paragraphs 52-56, wherein the        step of forming the p-type contact pad comprises depositing a        layer of p-type contact pad material, etching the p-type contact        pad material through and into the drift layer to form an etched        area, and depositing additional drift layer material to fill the        etched area.    -   58. A method of fabricating an electronic device of any of        paragraph 52 through paragraph 57, wherein the p-type contact        pad is formed at or below 800° C.    -   59. A method of fabricating an electronic device of any of        paragraph 52 through paragraph 58, wherein the p-type contact        pad is formed by pulsed laser deposition.    -   60. A method of fabricating an electronic device of any of        paragraph 52 through 59, wherein the drift layer is formed by        hydride vapor epitaxy.    -   61. A method of fabricating an electronic device of any of        paragraph 52 through paragraph 60, wherein the substrate is        fabricated by the ammonothermal method.    -   62. A method of fabricating an electronic device of any of        paragraph 52 through paragraph 61 further comprising dry etching        a portion of the drift layer before formation of the p-type        contact pad, and the device is not exposed to air between the        dry etching and the formation of the p-type contact pad.    -   63. A method according to any of paragraphs 52-62 and further        comprising forming an n-type contact pad on the substrate,        wherein the p-type contact pad is located closer to the        substrate than the n-type contact pad.    -   64. A method according to any of paragraphs 52-63 comprising        forming an n-type contact pad on the substrate at a position        sufficiently close to the p-type contact pad that a depletion        zone extends from said p-type contact pad and beneath the n-type        contact pad.

65. A method according to paragraph 64, wherein the n-type contact padis formed at a position sufficiently close to adjacent p-type contactpads that a depletion zone from each of the adjacent p-type contact padsextends beneath the n-type contact pad.

-   -   66. A method of fabricating an electronic device comprising:        -   (a) forming a drain layer of Ga_(1-x5-y5)Al_(x5)In_(y5)N            (0≤x5≤1, 0≤y5≤1) on a substrate of            Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1), wherein the            substrate has a resistivity of at least 1 kΩ cm and a            dislocation density less than 5×10⁵ cm⁻²;        -   (b) forming a drift layer of Ga_(1-x2-y2)Al_(x2)In_(y2)N            (0≤x2≤1, 0≤y2≤1) above the drain layer;        -   (c) forming a p-type layer of Ga_(1-x3-y3)Al_(x3)In_(y3)N            (0≤x3≤1, 0≤y3≤1) above the drift layer;        -   (d) etching the p-type layer and the drift layer but not the            drain layer to form a p-type contact pad and voids between            the p-type contact pads;        -   (e) depositing additional drift layer of            Ga_(1-x6-y6)Al_(x6)In_(y6)N (0≤x6≤1, 0≤y6≤1) in the voids;            and        -   (f) forming an Ohmic contact on the drain layer.    -   67. A method according to paragraph 66, wherein the method        further comprises forming an n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) above the drift        layer and above the p-type contact pad so that the p-type        contact pad is located closer to the substrate than the n-type        contact pad, and wherein the n-type contact pad is spaced        laterally apart from the p-type contact pad so that the n-type        contact pad does not touch the p-type contact pad.    -   68. A method according to paragraph 67, wherein the p-type        contact pad and the n-type contact pad are positioned        sufficiently close to one another on the substrate so that a        depletion region in the drift layer created by the p-type        contact pad prevents current flow from the n-type contact pad on        the drift layer to the Ohmic contact when no voltage is applied        between the n-type contact pad and the p-type contact pad.    -   69. A method according to any of paragraphs 66-68, wherein the        drift layer has an electron concentration less than 5×10¹⁶ cm⁻³.    -   70. A method according to any of paragraphs 66-69, wherein the        n-type contact pad is surrounded by the p-type contact pad.    -   71. A method according to paragraph 70, wherein the p-type        contact pad surrounds multiple n-type contact pads.    -   72. A method according to any of paragraphs 66-71, wherein the        step of etching the p-type layer to form the p-type contact pad        comprises etching along m-planes of the drift layer to form the        p-type contact pad.    -   73. A method according to any of paragraphs 67-72, wherein the        step of forming the n-type contact pad comprises aligning the        sides of the n-type contact pad to m-planes of the drift layer.    -   74. A method according to any of paragraphs 66-73, wherein the        p-type contact pad has a bandgap that is larger than a bandgap        of the drift layer.    -   75. A vertical electronic device comprising an n-type contact        pad, a p-type contact pad, a drift layer having a thickness, a        drain, and a substrate, wherein the drift layer is positioned        between the n-type contact pad and the drain so that an        electrical current flows from the n-type contact pad through the        thickness of the drift layer and to the drain, the n-type        contact pad being adjacent to the p-type contact pad, and        wherein the n-type contact pad and the p-type contact pad are        positioned sufficiently closely to one another on the substrate        so that a depletion region in the drift layer created by the        p-type contact pad prevents current flow from the n-type contact        pad on the drift layer to the Ohmic contact when no voltage is        applied between the n-type contact pad and the p-type contact        pad.    -   76. An electronic device of paragraph 75, wherein the substrate        has a first side and a second side opposite the first side, and        wherein the n-type contact pad, the p-type contact pad, the        drift layer, and the drain reside on the first side of the        substrate.    -   77. An electronic device of paragraph 75 or paragraph 76,        wherein the substrate has a first side and a second side        opposite the first side, and wherein the n-type contact pad, the        p-type contact pad, and the drift layer reside on the first side        of the substrate and wherein the drain resides on the second        side of the substrate.    -   78. An electronic device of any of paragraphs 75-77, wherein the        p-type contact pad has a bandgap that is larger than a bandgap        of the drift layer.    -   79. An electronic device of any of paragraphs 75-78, wherein        -   (a)the substrate has a dislocation density less than 5×10⁵            cm⁻²;        -   (b)the substrate has an electron concentration higher than            5×10¹⁸ cm⁻³;        -   (c)the drift layer has an electron concentration lower than            5×10¹⁶ cm⁻³; and        -   (d)the p-type contact pad has a hole concentration higher            than 1×10¹⁷ cm⁻³.    -   80. An electronic device of paragraph 79, wherein the n-type        contact pad is spaced apart from the p-type contact pad.    -   81. An electronic device of any of paragraphs 75-80, wherein the        p-type contact pad and the n-type contact pad are positioned        sufficiently close to one another on the substrate so that a        depletion region in the drift layer created by the p-type        contact pad prevents current flow from the n-type contact pad on        the drift layer to the Ohmic contact when no voltage is applied        between the n-type contact pad and the p-type contact pad.    -   82. An electronic device of any of paragraphs 75-81, wherein the        p-type contact pad is located closer to the substrate than the        n-type contact pad.    -   83. An electronic device of paragraph 75-82, wherein the n-type        contact pad is surrounded by the p-type contact pad.    -   84. An electronic device of paragraph 83, wherein all sides of        contact pads are aligned to m-plane of the drift layer.    -   85. An electronic device of any of paragraphs 75-84, wherein the        electron concentration of the n-type contact pad is more than        1×10¹⁸ cm⁻³.    -   86. An electronic device of any of paragraphs 75-85, wherein the        electron concentration of the drift layer is less than 1×10¹⁶        cm⁻³.    -   87. An electronic device of any of paragraphs 75-86, wherein the        substrate is Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1).    -   88. An electronic device of any of paragraphs 75-87, wherein the        p-type contact pad has a bandgap that is larger than a bandgap        of the drift layer.    -   89. An electronic device of paragraph 88, wherein the p-type        contact pad's bandgap is sufficiently greater than the drift        layer's bandgap to provide the electronic device with a hole        injection current from the p-type contact pad to the drift layer        that is more than 15,000 times higher than an electron injection        current from the drift layer to the p-type contact pad.    -   90. An electronic device of any of paragraphs 75-89, wherein the        substrate is made by an ammonothermal method and the substrate        has an electron concentration higher than 2×10¹⁹cm⁻³.    -   91. An electronic device of any of paragraphs 75-90, wherein a        carbon concentration of the drift layer is less than 1×10¹⁶        cm⁻³.    -   92. An electronic device comprising a substrate, an Ohmic        contact on one side of the substrate, a drift layer of        Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1) on an opposite side        of the substrate, at least one p-type contact pad of        Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) attached to the        drift layer, an Ohmic contact attached to the p-type contact        pad, and at least one n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer        spaced apart from the p-type contact pad, wherein:        -   (a) the substrate has a dislocation density less than 5×10⁵            cm⁻²;        -   (b) the substrate has an electron concentration higher than            5×10¹⁸ cm⁻³;        -   (c) the drift layer has an electron concentration lower than            5×10¹⁶ cm³¹ ³; and        -   (d) the p-type contact pad has a hole concentration higher            than 1×10¹⁷ cm⁻³.    -   93. An electronic device of paragraph 92, wherein the p-type        contact pad and the n-type contact pad are positioned        sufficiently close to one another on the substrate so that a        depletion region in the drift layer created by the p-type        contact pad prevents current flow from the n-type contact pad on        the drift layer to the Ohmic contact when no voltage is applied        between the n-type contact pad and the p-type contact pad.    -   94. An electronic device of paragraph 92 or paragraph 93,        wherein the p-type contact pad is located closer to the        substrate than the n-type contact pad.    -   95. An electronic device of any of paragraphs 92-94, wherein the        n-type contact pad is surrounded by the p-type contact pad.    -   96. An electronic device of paragraph 95, wherein all sides of        contact pads are aligned to m-plane of the drift layer.    -   97. An electronic device of any of paragraphs 92-96, wherein the        electron concentration of the n-type contact pad is more than        1×10¹⁸ cm⁻³.    -   98. An electronic device of any of paragraphs 92-97, wherein the        electron concentration of the drift layer is less than 1×10¹⁶        cm⁻³.    -   99. An electronic device of any of paragraphs 92-98, wherein the        substrate is Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1).    -   100. An electronic device of paragraph 99, wherein the substrate        is made by an ammonothermal method and the substrate has an        electron concentration higher than 2×10¹⁹cm⁻³.    -   101. An electronic device of any of paragraphs 92-100, wherein        the p-type contact pad has a bandgap that is larger than a        bandgap of the drift layer.    -   102. An electronic device of paragraph 101, wherein the p-type        contact pad's bandgap is sufficiently greater than the drift        layer's bandgap to provide the electronic device with a hole        injection current from the p-type contact pad to the drift layer        that is more than 15,000 times higher than an electron injection        current from the drift layer to the p-type contact pad.    -   103. An electronic device of paragraph 101 or paragraph 102        further comprising a p-GaN layer between the p-type contact pad        and the Ohmic contact to the p-type contact pad.    -   104. An electronic device of any of paragraphs 101-103 further        comprising at least one n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer        that is spaced a sufficient distance from the p-type contact pad        that the n-type contact pad is not directly touching the p-type        contact pad.    -   105. An electronic device of paragraph 104, wherein the        germanium concentration of the n-type contact pad is more than        1×10¹⁸ cm⁻³ and hydrogen concentration of the n-type contact pad        is less than 7×10¹⁶ cm⁻³.    -   106. An electronic device of paragraph 104 or paragraph 105,        wherein the electron concentration of the drift layer is low        enough to prevent electric current flowing from the Ohmic        contact on one side of the substrate to the n-type contact pad        when no voltage is applied between the n-type contact pad and        the p-type contact pad.    -   107. An electronic device of any of paragraphs 92-106, wherein        the p-type contact pad has a hydrogen concentration less than        one hundredth of a hydrogen concentration of the substrate.    -   108. An electronic device of paragraph 107, wherein the hydrogen        concentration of the p-type contact pad is less than one tenth        of a hydrogen concentration of the drift layer.    -   109. An electronic device of paragraph 107 or paragraph 108,        wherein the hydrogen concentration of the substrate is more than        5×10¹⁸ cm⁻³.    -   110. An electronic device of any of paragraphs 107-109, wherein        an oxygen concentration of the p-type contact pad is less than        one hundredth of an oxygen concentration of the substrate.    -   111. An electronic device of any of paragraphs 107-110, wherein        a silicon concentration of the p-type contact pad is less than        1×10¹⁸ cm⁻³.    -   112. An electronic device of any of paragraphs 107-111, wherein        the oxygen concentration of the substrate is more than 5×10¹⁸        cm⁻³.    -   113. An electronic device of any of paragraphs 107-112, wherein        a carbon concentration of the drift layer is less than 1×10¹⁶        cm⁻³.    -   114. An electronic device comprising a substrate of        Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1), a drain layer of        Ga_(1-x5-y5)Al_(x5)In_(y5)N (0≤x5≤1, 0≤y5≤1) and a drift layer        of Ga_(1-x2-y2)Al_(x2)In_(y2)N (0≤x2≤1, 0≤y2≤1) on one side of        the substrate, at least one p-type contact pad of        Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) attached to the        drift layer, an Ohmic contact attached to the p-type contact        pad, and at least one n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer        spaced apart from the p-type contact pad, wherein:        -   (a) the substrate has a dislocation density less than 5×10⁵            cm⁻²;        -   (b) the substrate has a resistivity of at least 1 kΩ cm;        -   (c) the drift layer has an electron concentration lower than            5×10¹⁶ cm⁻³; and        -   (d)the p-type contact pad has a hole concentration higher            than 1×10¹⁷ cm⁻³.    -   115. An electronic device of paragraph 114, wherein the p-type        contact pad is located closer to the substrate than the n-type        contact pad.    -   116. An electronic device of paragraph 114 or paragraph 115,        wherein the n-type contact pad is surrounded by the p-type        contact pad.    -   117. An electronic device of paragraph 116, wherein all sides of        contact pads are aligned to m-plane of the drift layer.    -   118. An electronic device of any of paragraphs 114-117, wherein        the p-type contact pad has a bandgap that is larger than a        bandgap of the drift layer.    -   119. An electronic device of any of paragraphs 114-118 and        further comprising a drain layer between the substrate and the        drift layer.    -   120. An electronic device of paragraph 119, wherein the drain        layer extends past the drift layer, and the drain layer has an        Ohmic contact on the same side of the substrate with the p-type        contact pad and the n-type contact pad.    -   121. An electronic device of paragraph 119 or paragraph 120,        wherein the p-type contact pad and the n-type contact pad are        positioned sufficiently close to one another on the substrate        that a depletion region in the drift layer created by the p-type        contact pad prevents current flow from the n-type contact pad on        the drift layer to the drain layer's Ohmic contact when no        voltage is applied between the n-type contact pad and the p-type        contact pad.    -   122. A method of fabricating an electronic device comprising:        -   (a) forming a drain layer of Ga_(1-x5-y5)Al_(x5)In_(y5)N            (0≤x5≤1, 0≤y5≤1) on a substrate of            Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1), wherein the            substrate has a resistivity of at least 1 kΩcm and a            dislocation density less than 5×10⁵ cm⁻²;        -   (b) forming a drift layer of Ga_(1-x2-y2)Al_(x2)In_(y2)N            (0≤x2≤1, 0≤y2≤1) above the drain layer;        -   (c) forming a p-type layer of Ga_(1-x3-y3)Al_(x3)In_(y3)N            (0≤x3≤1, 0≤y3≤1) above the drift layer;        -   (d) etching the p-type layer and the drift layer but not the            drain layer to form a p-type contact pad and voids between            the p-type contact pads;        -   (e) depositing additional drift layer of            Ga_(1-x6-y6)Al_(x6)In_(y6)N (0≤x6≤1, 0≤y6≤1) in the voids;            and        -   (f) forming an Ohmic contact on the drain layer.    -   123. A method according to paragraph 122, wherein the method        further comprises forming an n-type contact pad of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) above the drift        layer and above the p-type contact pad so that the p-type        contact pad is located closer to the substrate than the n-type        contact pad, and wherein the n-type contact pad is spaced        laterally apart from the p-type contact pad so that the n-type        contact pad does not touch the p-type contact pad.    -   124. A method according to paragraph 123, wherein the p-type        contact pad and the n-type contact pad are positioned        sufficiently close to one another on the substrate so that a        depletion region in the drift layer created by the p-type        contact pad prevents current flow from the n-type contact pad on        the drift layer to the Ohmic contact when no voltage is applied        between the n-type contact pad and the p-type contact pad.    -   125. A method according to paragraph 123 or paragraph 124,        wherein the step of forming the n-type contact pad comprises        aligning the sides of the n-type contact pad to m-planes of the        drift layer.    -   126. A method according to any of paragraphs 122-125, wherein        the drift layer has an electron concentration less than 5×10¹⁶        cm⁻³.    -   127. A method according to any of paragraphs 122-126, wherein        the n-type contact pad is surrounded by the p-type contact pad.    -   128. A method according to paragraph 127, wherein the p-type        contact pad surrounds multiple n-type contact pads.    -   129. A method according to any of paragraphs 122-128, wherein        the step of etching the p-type layer to form the p-type contact        pad comprises etching along m-planes of the drift layer to form        the p-type contact pad.    -   130. A method according to any of paragraphs 122-129, wherein        the p-type contact pad has a bandgap that is larger than a        bandgap of the drift layer.    -   131. A method of fabricating an electronic device comprising:        -   (a) growing a drift layer of Ga_(1-x2-y2)Al_(x2)In_(y2)N            (0≤x2≤1, 0≤y2≤1) by vapor phase epitaxy on a first side of a            substrate of Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1);        -   (b)forming a p-type contact pad of            Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) on the drift            layer by a deposition method which does not use a            hydrogen-containing source;        -   (c) forming an Ohmic contact on a second side of the            substrate.    -   132. The method of paragraph 131, wherein the step of growing        the drift layer by vapor phase epitaxy does not use a        carbon-containing source.    -   133. The method paragraph 131 or paragraph 132 further        comprising forming n-type contact pads of        Ga_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer        by a deposition method which does not use a hydrogen-containing        source, and wherein the n-type contact pads are separated from        the p-type contact pads by a sufficient distance that the n-type        contact pads do not directly touch the p-type contact pads.    -   134. The method of paragraph 133 further comprising etching a        trench in the drift layer to make the p-type contact pad closer        to the substrate than the n-type contact pads are to the        substrate.    -   135. The method of paragraph 134, wherein the p-type contact pad        is formed without exposing the device to air after etching the        trench in the drift layer.    -   136. The method of any of paragraphs 131-135, wherein the step        of forming the p-type contact pad comprises depositing a layer        of p-type contact pad material, etching the p-type contact pad        material through and into the drift layer to form an etched        area, and depositing additional drift layer material to fill the        etched area.    -   137. The method of any of paragraphs 131-136, wherein the p-type        contact pad is formed at or below 800° C.    -   138. The method of any of paragraphs 131-137, wherein the p-type        contact pad is formed by pulsed laser deposition.    -   139. The method of any of paragraphs 131-138, wherein the drift        layer is formed by hydride vapor epitaxy.    -   140. The method of any of paragraphs 131-139, wherein the        substrate is fabricated by the ammonothermal method.    -   141. The method of any of paragraphs 131-140 further comprising        dry etching a portion of the drift layer before formation of the        p-type contact pad, and the device is not exposed to air between        the dry etching and the formation of the p-type contact pad.    -   142. The method of any of paragraphs 131-141 and further        comprising forming an n-type contact pad on the substrate,        wherein the p-type contact pad is located closer to the        substrate than the n-type contact pad.    -   143. The method of any of paragraphs 131-142 comprising forming        an n-type contact pad on the substrate at a position        sufficiently close to the p-type contact pad that a depletion        zone extends from said p-type contact pad and beneath the n-type        contact pad.    -   144. The method of paragraph 143, wherein the n-type contact pad        is formed at a position sufficiently close to adjacent p-type        contact pads that a depletion zone from each of the adjacent        p-type contact pads extends beneath the n-type contact pad.

Possible Modifications

Although the preferred embodiment describes GaN substrates, thesubstrate can be group III nitride alloys of various composition, suchas AlN, AlGaN, InN, InGaN, or GaAlInN. The scope of the inventionincludes these substrates.

Although the preferred embodiment describes Ga-face c-plane GaN, otherorientations such as N-face c-plane, a-face, m-face, and varioussemipolar surface can also be used. In addition, the surface can beslightly miscut (off-sliced) from these orientations. The scope of theinvention includes these orientations and miscut. In particular, usageof N-face c-plane GaN, nonpolar a-face and m-face, semipolar planes maymodulate the energy band structure of the electronic devices, and thuscould control the turn-on voltage.

Although the preferred embodiment describes HVPE or MOCVD as a vaporphase epitaxy, other methods such as MBE, reactive sputtering, andion-beam deposition can be used for growing the active layer and/or thetransition layer in this invention.

Although the preferred embodiment uses Ni/Au for p-type Ohmic contactand Ti/Al for n-type Ohmic contact, other materials such as In, ZnO, andW can be used.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

REFERENCES

The following references are incorporated by reference herein:

[1] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 6,656,615.

[2] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 7,132,730.

[3] R. Dwiliński, R. Doradziński, J. Garczński, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 7,160,388.

[4] K. Fujito, T. Hashimoto, S. Nakamura, International PatentApplication No. PCT/US2005/024239, WO07008198.

[5] T. Hashimoto, M. Saito, S. Nakamura, International PatentApplication No. PCT/US2007/008743, WO07117689. See also US20070234946,U.S. application Ser. No. 11/784,339 filed Apr. 6, 2007.

[6] D′Evelyn, U.S. Pat. No. 7,078,731.

Each of the references above is incorporated by reference in itsentirety as if put forth in full herein, and particularly with respectto description of methods of making using ammonothermal methods andusing these gallium nitride substrates.

What is claimed is:
 1. A method of fabricating an electronic devicecomprising: (a) growing a drift layer of Ga_(1-x2-y2)Al_(x2)In_(y2)N(0≤x2≤1, 0≤y2≤1) by vapor phase epitaxy on a first side of a substrateof Ga_(1-x1-y1)Al_(x1)In_(y1)N (0≤x1≤1, 0≤y1≤1); (b)forming a p-typecontact pad of Ga_(1-x3-y3)Al_(x3)In_(y3)N (0≤x3≤1, 0≤y3≤1) on the driftlayer by a deposition method which does not use a hydrogen-containingsource; (c) forming an Ohmic contact on a second side of the substrate.2. The method of claim 1, wherein the step of growing the drift layer byvapor phase epitaxy does not use a carbon-containing source.
 3. Themethod claim 1 further comprising forming n-type contact pads ofGa_(1-x4-y4)Al_(x4)In_(y4)N (0≤x4≤1, 0≤y4≤1) on the drift layer by adeposition method which does not use a hydrogen-containing source, andwherein the n-type contact pads are separated from the p-type contactpads by a sufficient distance that the n-type contact pads do notdirectly touch the p-type contact pads.
 4. The method of claim 3 furthercomprising etching a trench in the drift layer to make the p-typecontact pad closer to the substrate than the n-type contact pads are tothe substrate.
 5. The method of claim 4, wherein the p-type contact padis formed without exposing the device to air after etching the trench inthe drift layer.
 6. The method of claim 1, wherein the step of formingthe p-type contact pad comprises depositing a layer of p-type contactpad material, etching the p-type contact pad material through and intothe drift layer to form an etched area, and depositing additional driftlayer material to fill the etched area.
 7. The method of claim 1,wherein the p-type contact pad is formed at or below 800° C.
 8. Themethod of claim 1, wherein the p-type contact pad is formed by pulsedlaser deposition.
 9. The method of claim 1, wherein the drift layer isformed by hydride vapor epitaxy.
 10. The method of claim 1, wherein thesubstrate is fabricated by the ammonothermal method.
 11. The method ofclaim 1 further comprising dry etching a portion of the drift layerbefore formation of the p-type contact pad, and the device is notexposed to air between the dry etching and the formation of the p-typecontact pad.
 12. The method of claim 1 and further comprising forming ann-type contact pad on the substrate, wherein the p-type contact pad islocated closer to the substrate than the n-type contact pad.
 13. Themethod of claim 1 comprising forming an n-type contact pad on thesubstrate at a position sufficiently close to the p-type contact padthat a depletion zone extends from said p-type contact pad and beneaththe n-type contact pad.
 14. The method of claim 13, wherein the n-typecontact pad is formed at a position sufficiently close to adjacentp-type contact pads that a depletion zone from each of the adjacentp-type contact pads extends beneath the n-type contact pad.